This invention relates to a method for preparing an organoxy-terminated organopolysiloxane useful as a base polymer in a one-part organopolysiloxane elastomer composition which is stable during storage in the absence of moisture and crosslinks with air-borne moisture at the ambient temperature.
It is well known in the art to prepare organoxy-terminated organopolysiloxanes by reacting di-, tri- or tetraalkoxysilanes with hydrosilyl groups attached to silicon atoms at molecular ends in the presence of catalysts.
The use of amine catalysts is disclosed in U.S. Pat. No. 3,542,901, U.S. Pat. No. 4,489,191, French Patent No. 2,597,876, and French Patent No. 2,597,877. U.S. Pat. No. 3,504,051 discloses the use of potassium acetate, French Patent No. 1,495,011 discloses the use of inorganic oxides, U.S. Pat. No. 3,647,846 discloses the combination of titanates and amines, French Patent No. 2,604,713 discloses the combination of carboxylic acids and amines, EP 0210402 discloses the use of carbamates, French Patent No. 2,597,875 discloses the use of oxime functional group-containing organic compounds, and JP-B 6-45698 discloses the use of lithium hydroxide.
However, the use of amine catalysts has many problems including a reaction time as long as about 15 to 30 minutes at a heating temperature of 60xc2x0 C., a low percent end blockage, an increased amount of residual SiOH, yellowing and storage instability due to the residual amine which is difficult to remove.
The foregoing methods use acids or bases and need a neutralizing step. Although the products are now required to be neutral; the use of amine catalysts suffers from a high probability of leaving amines in the products.
Additionally, U.S. Pat. No. 4,111,890 describes the use of organic titanium derivatives, and British Patent No. 2,133,758 describes the use of alkoxyaluminum chelates. The aluminum and titanium chelates used therein are adequate as the condensation curing catalyst, but not for end blockage because they promote crosslinking during reaction, resulting in gelation and substantial thickening.
Therefore, an object of the invention is to provide a method for preparing an organoxy-terminated organopolysiloxane in an efficient manner without gelation or other troubles and without a need for neutralization.
The invention provides a method for preparing an organoxy-terminated organopolysiloxane, comprising the step of reacting (1) an organopolysiloxane of the following general formula (I): 
wherein R is a monovalent hydrocarbon group and n is such an integer that the organopolysiloxane has a viscosity of 10 to 100,000 centistokes at 25xc2x0 C., with (2) an organoxysilane of the following general formula (II): 
wherein xe2x80x9caxe2x80x9d is an integer of 0, 1 or 2, R1 is a substituted or unsubstituted monovalent hydrocarbon group of 1 to 18 carbon atoms, and R2 is an organic group of 1 to 18 carbon atoms, or a partial hydrolyzate thereof, to form an organoxy-terminated organopolysiloxane of the general formula (A): 
wherein R, R1, R2, xe2x80x9caxe2x80x9d and n are as defined above. The invention is characterized in that the reaction of components (1) and (2) is effected in the presence of (3) a tetraalkoxytitanium of the following general formula (III):
Ti(OR3)4xe2x80x83xe2x80x83(III)
wherein R3 is an alkyl group as a catalyst. The organoxy-terminated organopolysiloxane of formula (A) is prepared within one hour even at room temperature and usually, at an end blockage of at least 85%. By adding methanol to a mixture of components (1), (2) and (3) during reaction or at the end of reaction, the end blocking time is significantly reduced.
The method is advantageous in that since neither basic nor acidic catalysts are used, the resulting organoxy-terminated organopolysiloxane is suited in electric and electronic applications. In the presence of moisture, this organopolysiloxane will readily cure into a rubbery elastomer. A room temperature curable composition comprising this organopolysiloxane as a base polymer is useful in a wide variety of applications such as adhesives, coating agents, electrically insulating sealants, and building sealants.
The method for preparing an organoxy-terminated organopolysiloxane according to the invention starts with (1) an organopolysiloxane and (2) an organoxysilane. Component (1) is an organopolysiloxane of the following general formula (I). 
Herein R is a monovalent hydrocarbon group and n is such an integer that the organopolysiloxane has a viscosity of 10 to 100,000 centistokes at 25xc2x0 C.
R is selected from substituted or unsubstituted monovalent hydrocarbon groups preferably of 1 to 10 carbon atoms, and more preferably 1 to 8 carbon atoms, for example, alkyl groups such as methyl, ethyl, propyl, butyl and hexyl, aryl groups such as phenyl and tolyl, alkenyl groups such as vinyl, allyl and butenyl, aralkyl groups such as benzyl and 2-phenylethyl, and substituted ones of the foregoing groups in which some or all of the hydrogen atoms attached to carbon atoms are replaced by halogen atoms and cyano groups, such as chloromethyl, trifluoropropyl and cyanoethyl. Of these, methyl, phenyl, vinyl and trifluoropropyl are preferred, with methyl being most preferred.
The letter n representative of the degree of polymerization is such an integer that the organopolysiloxane has a viscosity of 10 to 100,000 centistokes at 25xc2x0 C. and preferably 500 to 100,000 centistokes at 25xc2x0 C. for ease of working.
Illustrative, non-limiting examples of the organopolysiloxane of formula (I) are given below. 
In the formulas, Me is methyl, Ph is phenyl, p and q are positive integers, p+q is an integer corresponding to n. Of these, the compounds of formula (I-a) are preferred.
In addition to the above-described structure, the polymer may include a structure which is blocked at one end with 0 to 10 mol % of trimethylsilyl groups. Also the polymer may contain 0 to 3 mol % based on its molecular weight of a branched structure as shown by the following structural formula. 
The other starting reactant, component (2) serves as an end blocking agent for component (1) and is an organoxysilane of the following general formula (II) or a partial hydrolyzate thereof. 
Herein xe2x80x9caxe2x80x9d is an integer of 0, 1 or 2, and preferably 0 or 1, R1 is a substituted or unsubstituted monovalent hydrocarbon group of 1 to 18 carbon atoms, and R2 is an organic group of 1 to 18 carbon atoms.
Examples of R1 are alkyl, alkenyl, aryl and aralkyl groups and halo- and cyano-substituted groups thereof as exemplified above for R, with alkyl, alkenyl and aryl groups being preferred.
R2 is preferably selected from aliphatic organic groups of 1 to 18 carbon atoms, for example, alkyl, alkenyl, aryl, alkyl ether, alkyl ester, alkyl ketone and alkylcyano groups. R2 is preferably an alkyl group, more preferably alkyl group of 1 to 4 carbon atoms, and most preferably methyl.
The preferred organoxysilane is an alkoxysilane of the following general formula (II-a):
R1bSi(OCH3)4-bxe2x80x83xe2x80x83(II-a)
wherein R1 is as defined above and b is equal to 0 or 1, or a partial hydrolyzate thereof.
Illustrative, non-limiting examples of the organoxysilane include n-propyltrimethoxysilane, methyltrimethoxysilane, vinyltrimethoxysilane, tetramethoxysilane, ethyltrimethoxysilane, and phenyltrimethoxysilane. For ease of handling, vinyltrimethoxysilane and n-propyltrimethoxysilane are especially preferred. In the batchwise preparation where reaction heat accumulates, n-propyltrimethoxysilane is especially preferred because of its low volatility.
Preferably components (1) and (2) are used in such amounts that the moles of component (2) is at least equal to the moles of component (1) or silanol. More preferably the ratio of the moles of component (1) to the moles of component (2) is from 1:1 to 1:100 and especially from 1:1 to 1:20. Using component (2) in excess is effective for reducing the time for ends to be blocked and yielding a high percent end blockage. A molar ratio within the above range permits a continuous process to be employed with better results.
By reacting components (1) and (2), there is prepared an organoxy-terminated organopolysiloxane of the general formula (A): 
wherein R, R1, R2, xe2x80x9caxe2x80x9d and n are as defined above.
According to the invention, the reaction of components (1) and (2) is effected in the presence of (3) a tetraalkoxytitanium or titanium tetraalkoxide catalyst of the following general formula (III):
Ti(OR3)4xe2x80x83xe2x80x83(III)
wherein R3 is an alkyl group, preferably of 1 to 19 carbon atoms, and especially 2 to 4 carbon atoms.
Any titanium tetraalkoxide may be used only for end blocking purposes. The end blocking rate is faster in the order of titanium tetraethoxide greater than titanium tetra-n-propoxide greater than titanium tetraisopropoxide greater than titanium tetrabutoxide. Of these, titanium tetraethoxide is most preferred although titanium tetraisopropoxide is most readily available in the industry.
Component (3) is preferably used in such amounts that the ratio of the moles of component (3) to the moles of silanol (1) is from 0.001 to 20, more preferably from 0.01 to 10, and especially 0.05 to 5. Outside the range, the catalyst may become ineffective.
According to the invention, components (1) to (3) are admitted into a suitable reactor whereupon reaction proceeds at room temperature or elevated temperature. Preferably the reaction is effected under pressure. In this regard, a reactor capable of tight closure so that pressure is naturally applied is preferred because the reaction is promoted thereby. Although a higher pressure is preferable, an upper limit of up to 10 atm. and especially up to 5 atm. is recommended for safety. The lower limit is not critical as long as the pressure is higher than atmospheric, although it is at least 1.05 atm. and especially at least 1.1 atm.
According to the invention, reaction is effected by mixing components (1) to (3) under atmospheric pressure or under applied pressure. More specifically, component (1) first reacts with component (3) and thus thickens. Then component (2) substitutes for component (3) whereupon the desired organoxy-terminated organopolysiloxane is obtained. Although the viscosity is once increased, the product resumes the original or desired viscosity at this point.
In the preferred embodiment, methanol is added to the reaction system after it has thickened by reaction of component (1) with component (3). By adding methanol, the time for end blockage with organoxy groups can be significantly reduced. If methanol is added at the initial, there is a potential for methanol to stop the end blocking reaction. It is therefore preferred to add methanol after component (1) has reacted with component (3) to incur a viscosity increase and before the end blocking reaction with component (3) proceeds to completion. Even when methanol is added at the substantial end of reaction, the percent end blockage can be further increased.
An appropriate amount of methanol added is about 0.1 to 20 parts, and especially about 0.2 to 10 parts by weight per 100 parts by weight of component (1). Less amounts of methanol are ineffective whereas excessive amounts may rather restrain the end blocking reaction. Since there is a situation where the viscosity changes a little even after the completion of reaction, mixing for deaeration under vacuum or atmospheric pressure is recommended.
The organopolysiloxane of formula (A) is useful as a base polymer in a room temperature curable organopolysiloxane composition of the one part type. To the organopolysiloxane, there may be added a filler, for example, surface treated or untreated silica, heavy calcium carbonate, surface treated or untreated colloidal calcium carbonate; a curing agent, for example, a compound having at least two hydrolyzable groups; an adhesive aid as typified by aminosilane; a condensation catalyst, and other additives. By adding these additives during the mixing step for deaeration, there is obtained a room temperature curable organopolysiloxane composition of the one part type. The room temperature curable organopolysiloxane composition comprising the organoxy-terminated organopolysiloxane according to the invention remains stable during storage, is strongly adhesive to adherends, especially surface treated aluminum members, and cures into a product maintaining a firm bond against water immersion and heating. The room temperature curable composition is thus useful in a wide variety of applications such as adhesives, coating agents, electrically insulating sealants, and building sealants.